1. Field of the Invention
The present invention relates to a color filter applicable to color liquid crystal display devices used in color televisions, personal computers, navigation systems for automobile, etc., a production process thereof, a liquid crystal display device using this color filter and a process for the production of black matrix.
2. Related Background Art
A black matrix called a light-screening film has heretofore been incorporated in a color filter for liquid crystal display device for the purpose of preventing color mixing between R (red), G (green) and B (blue) colors and enhancing contrast. The black matrix has heretofore been formed by patterning a film of a metal such as chromium using a photolithographic process. The black matrix formed by using this process has a high reflectance. Therefore, a color filter in which such a black matrix has been incorporated is high in light reflectance and a problem has hence arisen as to the visibility of images.
It has therefore been proposed to realize low reflecting property by sandwiching a metal chromium film between oxide layers such as chromium oxide films. A black matrix formed by such metal and metal compound has had merits that it is high in absorption constant against visible rays, thin in film thickness and high in optical density.
In view of environmental problems in recent years, however, chromium has not only have a possibility that it may change to chromium (VI) injurious to the human body, but also involved a problem that a heavy metal-containing oxidizing agent such as ammonium cerium (IV) nitrate must be used.
In order to solve these problems, resinous black matrix materials of the negative black resist type have been proposed in recent years. Such a black resist may be developed with a dilute solution of alkali such as sodium carbonate and has no possibility that it may change to a substance injurious to the human body when it is left to stand in the natural world.
As the resinous black matrix material, there is generally used that obtained by dispersing a black pigment such as carbon black having a particle diameter of 0.2 .mu.m or smaller in an alkali-soluble type negative resist. Since this resinous black matrix contains the black pigment, light energy does not reach the lower part of the resist material upon exposure to light. Accordingly, a photo-set portion by the exposure is limited to a surface layer portion of the resist layer. Therefore, the whole resist layer is set by a heat treatment or the like after development.
The resinous black matrix is lower in absorption coefficient compared with the metallic black matrix, and so the film thickness must be thickened in order to obtain the prescribed optical density. When the film thickness is thickened, however, the sectional form of the black matrix has not become an ideal rectangle in some cases as illustrated in FIGS. 4 and 5. These drawings each illustrate the sectional form of a black matrix right after development. In the drawings, reference numeral 1 indicates a transparent substrate, 2a is an unset portion of a black resist layer, and 2b is a set portion of the black resist layer. FIG. 4 shows a case where side etching has occurred at the unset portion 2a upon the development, while FIG. 5 shows a case where the set portion 2b formed in a reverse-tapered shape by the side etching at the unset portion 2a has become fragile and chipped.
When the sectional form becomes such a form as illustrated in FIG. 4 or 5, the optical path length of light transmitted through the edge portion of the black matrix becomes shorter than that of light transmitted through a portion having the prescribed thickness. Therefore, such light is not fully absorbed in the black matrix. Accordingly, in a color filter formed by using such a black matrix, the edges of the black matrix blur, and disadvantages such as lowering of contrast and the occurrence of blank areas in color pixels arise.
When the set portion 2b weakened by the occurrence of the side etching chips in a subsequent step, the chipped set portion forms the cause of generation of dust or particles.
In order to solve such a problem, Japanese Patent Application Laid-Open No. 7-248412 discloses a two-layer film composed of a black resist and a chromium film. In this method, after the black resist is exposed to light, the chromium film is formed, and thereafter an exposed portion or unexposed portion is developed to lift off the chromium film. A black matrix in a reverse-tapered shape may be formed according to the conditions of the development, and moreover the chromium film formed on the set portion can also prevent the set portion from chipping to generate dust. In addition, even when the black matrix is formed in the reverse-tapered shape, disadvantages such as blurring of the edges thereof do not arise because the light-screening property of chromium is high. Since the resist is developed to lift off the chromium film, there is also no need to use an expensive developing solution for directly etching the chromium film.
However, such a method requires an expensive sputtering apparatus for forming the chromium film. In addition, when a chromium oxide film is intended to form by reactive sputtering for the purpose of enhancing the adhesion of the resulting chromium film upon the formation of the chromium film by the sputtering or providing a low-reflective chromium film, it is general to conduct heating. However, such heating is infeasible, since a resist is thermoset by the heating when the resist is formed, and such a chromium film is then formed thereon.
Accordingly, there has been a demand for development of a process for producing a resinous black matrix with good precision at a low cost.